1. Field of the Invention
The present invention relates to a lighting apparatus, and in particular relates to a lighting apparatus for observing patterns such as circuitry, letters and the like which are formed on the surface or in the vicinity of the surface of an insulating substrate such as ceramic, glass, a semiconductor wafer or the like.
2. Description of the Prior Art
Up to now, during the manufacture of semiconductor integrated circuits and the like, microscopes, cameras and even the human eye have been used to observe patterns such as circuitry, letters and numbers which have been formed on the surface of a material such as resin, ceramic, glass, a liquid crystal substrate, a semiconductor wafer or the like. For example, during a manufacturing process, identification marks on a semiconductor wafer are read out, and then a predetermined process is carried out in accordance with such identification marks.
Several lighting apparatuses are known in the prior art for illuminating an object to be observed, and these apparatuses generally employ fluorescent lights, fiber optic illumination or a parallel light source comprised of lenses or the like.
When reading patterns such as letters or the like on a substrate, if there is a sufficiently large enough contrast between the pattern and the surrounding background, such pattern will be clearly seen, but if there is little contrast between the pattern and the surrounding background, it will be difficult to distinguish the pattern from the surrounding background. Such contrast between the pattern and the surrounding background is affected not only by the illuminance of the light source but also by the illuminating angle and the illuminating angle distribution of the light source. "Illuminating angle distribution" means the distribution of illuminating angles within an observed area.
Although there is a bright field illumination for illuminating dark patterns on light backgrounds and a dark field illumination for illuminating light patterns on dark backgrounds, it is not necessarily a good idea to always employ a bright field illumination having a high illuminance.
Thus, in the lighting apparatuses of the prior art, it is necessary to consider the light source, the optical elements, the position of the object to be observed and the observer when determining the arrangement of the observation system in order to obtain a sufficiently large contrast at the portion being observed. Once this is done, it is then necessary to fix the positional relationship of such determined observation system; namely it becomes necessary to fix the illuminating angle and the illuminating angle distribution of the light which shines onto the object to be observed. In this regard, an operator must have sufficient experience and time to finely adjust the illuminating angle and the illuminating angle distribution.
In this connection, Japanese Laid-Open Patent Publication Nos. 6-3625 & 6-129844 disclose a semiconductor wafer inspection apparatus in which parallel light is shined onto a sample, with the reflected light or scattered light therefrom being focused by a convex lens toward an aperture stop arranged within the back focal plane of the convex lens. However, even in this type of apparatus, the illuminating angle and the illuminating angle distribution of the light are fixed, and in order to finely adjust such illuminating angle and illuminating angle distribution, an operator must have sufficient experience and time.
Furthermore, even when observations are carried out using the same degree of illumination, there will be a wide variation in contrast due to the variation in optical characteristics of the object being observed as a result of differences in the manufacturing site, manufacturing apparatus or manufacturing process. In particular, there will be a wide variation in reflectance and transmittance due to warping of the object being observed and unevenness in the thickness of the pattern and surrounding material. Consequently, there will arise situations in which it is not possible to observe patterns due to low levels of contrast.
In such cases, for each observation, an optical axis adjustment which requires sufficient experience is carried out to obtain a high contrast for the object being observed. However, such adjustments require a great deal of time, and depending on the situation, there are many times in which an object must be treated as being unobservable.
To better understand this problem, a detailed description will be given for a sample semiconductor wafer. Namely, a semiconductor wafer has ID letters (identification marks) written into the top thereof by means of a write-in method, such as the same method used to form an integrated circuit (IC) pattern by means of photographic exposure or a method which employs a high-powered laser to etch letters into the top of the wafer. In general, ID letters are written on the wafer before the IC pattern is formed, after which each wafer undergoes a predetermined process in accordance with the ID letters thereof.
When such predetermined processes are carried out, a group of wafers called a "lot" undergo various treatments (vapor deposition, putting resist, exposure, etching, etc.). In this regard, it is quite common for each lot to undergo different treatments, and different processes need to be carried out depending on the type of IC being formed. Now, because such treatments are u ed to form several thin films on top of the wafer, various problems can arise as the processes involved in these treatments are carried out, such as warping of the wafer, the formation of uneven layers due to irregular resist in the vicinity of the ID letters, and damage to the wafer when conveyed from one location to another. These problems accumulate as the processes are carried out.
Now, in the prior art lighting apparatus, when a camera is used to observe the ID letters on top of a wafer which has undergone the various treatments described above, it is necessary to make the above-mentioned optical axis adjustments for each lot, each IC type and each ID writing method. In the case where it is not possible to carry out the adjustments, the light noise components due to irregular resist of the areas surrounding the ID letters combined with defects of the wafer will give rise to so-called image noise, and this can make it impossible to obtain the image of the ID letters. As a result, some cases need to be treated as unobservable.